Transitions in refractive index using electro-optic polymers

ABSTRACT

The index of refraction in a length of doped and/or “doped-and-poled” electro-optic polymers is controlled so that a gradual transition from a low Δn to a high Δn, or vice versa, is achieved for use in, for example, a lightguide-to-fiber transition. Multiple methods for creating this gradual transition are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of prior filed U.S. provisionalApplication No. 60/408,763, filed on Sep. 6, 2002, incorporated fullyherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for coupling lightguides to fiberoptic transmission lines.

2. Description of the Related Art

In the electro-optic or photonics industry, it is frequently necessaryto couple lightguides with fiber optic transmission lines, e.g., whencoupling an optical component on a circuit board to a light pipe. Amajor problem when coupling lightguides to fiber optic transmissionlines is that typically lightguides have a high “refractive indexdifference” (Δn) percentage between the core and the cladding ascompared to the Δn of fiber optic transmission lines. A typical on-chipor on-circuit board planar lightguide has a Δn of 4-5%, while a typicalfiber optic transmission line has a Δn of approximately 0.5-2%. Thissignificant drop of Δn between the lightguide and the transmission linecauses problem, e.g., reflection resulting in a reduction in the amountof light that can be transmitted. To solve this problem, opticalcoupling techniques have been used to ease the transition from thislow-to-high or high-to-low Δn's, for example, lenses are used to focus(or spread) the light, making the transition more gradual. Whilefunctioning adequately, prior art techniques are custom solutions thatare costly and that take up significant space on a circuit board orother location where space is at a premium.

SUMMARY OF THE INVENTION

In accordance with the present invention, the index of refraction indoped and/or “doped-and-poled” electro-optic polymers is controlled sothat a gradual transition from a low Δn to a high Δn, or vice versa, isachieved for use in, for example, a lightguide-to-fiber transition.Multiple methods for creating this gradual transition are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 c illustrate top, end, and side views, respectively, of alightguide-to-optical-transmission-line coupler in accordance with thepresent invention;

FIG. 2 is an index profile showing the change in the Δn values ofregions 1-5 of FIGS. 1 a-1 c;

FIGS. 3 a-3 c illustrate top, end, and side views, respectively, of alightguide-to-optical-transmission-line coupler in accordance with analternative embodiment of the present invention;

FIG. 4 illustrates the relationship of the time of the exposure of acladding layer to doped source, to the Δn values with respect to theembodiment of FIGS. 3 a-3 c;

FIGS. 5 a-5 f illustrates the selective removal process of theembodiment of FIGS. 3 a-3 c;

FIGS. 6 a-6 c illustrate top, end, and side views, respectively, of acoupler which has formed thereon separated poling electrodes accordingto another embodiment of the present invention;

FIG. 7 illustrates the resulting gradually increasing Δn value of theembodiment of FIGS. 6 a-6 c;

FIGS. 8 a-8 c illustrate a common reference electrode and common polingelectrode used to sandwich a tapered doped material according to anotherembodiment of the present invention; and

FIG. 9 illustrates the decreasing Δn that results from applying aconstant poling voltage to a doping material of increasing smallerthickness.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The overall concept of the present invention is the controlled changingof the index of refraction of doped and/or doped-and-poled electro-opticpolymers from one region of the polymer to the next. This control can beperformed by controlling the optical chromophore doping (percentage ofchromophore by weight) of the polymer and the temperature of the polymercure. In a doped-and-poled polymer, the strength of the poling field isalso a major variable, coupled with the doping percentage and, to somedegree, the temperature at which the poling process takes place. Boththe doping and the amount of poling can be selectively accomplished bymasking and the strategic placement of poling electrodes using knowntechniques.

By changing gradually the index of refraction along a connection betweena lightguide and a fiber optic transmission line, the sudden change inthe index of refraction between the lightguide and the fiber optictransmission line is alleviated, thereby alleviating the problemsassociated therewith. Several embodiments are described below forperforming this gradual transition.

FIGS. 1 a-1 c and 2 illustrate a first embodiment of the presentinvention. FIGS. 1 a-1 c illustrate top, end, and side views,respectively, of a lightguide-to-optical-transmission-line coupler inaccordance with the present invention. Referring to FIG. 1, a coupler100 is shown having a core 102 and doping regions 1-5 on either sidethereof. The high index core comprises an undoped polymer and the dopingregions 1-5 are regions of increasingly heavier doping depositedsequentially along the length (on both sides) of the high index core 102as shown. The lowest doped region is region 1 and the highest dopedregion is doping region 5 with varying levels of increasing dopingoccurring in doping regions 2, 3, and 4, respectively. The regions ofincreased doping can be formed by, for example, selective depositionthrough a protective masking layer that is patternedphotolithographically. The actual number of depositions (or regions) isnot limited to five and varies depending upon the required Δn shift;thus, there may be greater or fewer doping regions, depending upon need.If the doping regions are large, shadow masking may be used. Polymerdeposition can be accomplished by spraying, dipping, vacuum pyrolysis,or any other known deposition means which will produce the graduallyincreasing doped regions.

FIG. 2 is an index profile showing the change in the Δn values ofregions 1-5 of FIGS. 1 a-1 c. As can be seen, with each doping region,beginning with doping region 1, the Δn value increases incrementallyfrom a low Δn at doping region 1 to the highest Δn at doping region 5.

By providing a coupler 100 with these characteristics, a fiber optictransmission line can be coupled at the end closest to region 1 (so thatthe low Δn percentage of the fiber optic transmission line will be closein value to that of region 1 of the coupler 100), and the lightguide canbe coupled to region 5 of coupler 100 (so that the higher Δn value ofthe lightguide is more closely matched to region 5 of coupler 100)thereby resulting in a gradual transition from lightguide totransmission line.

FIGS. 3-5 illustrate an alternative embodiment of the present invention,in which doping regions 1-5 of increasing heavier doping are formed bydiffusion doping of an undoped polymer (core 302 in FIG. 3 a) forvarying periods of time. Using this method, an impurity source 520(FIGS. 5 a-5 f), such as a very heavily doped polymer film, is depositedon top of the entire region 1-5 (the region to receive the progressivelyincreasing doping levels). Impurity source 520 is then selectivelyremoved with time to create the increasing doped density, by positionalong the core, in the cladding layer. The selective removal process isillustrated in FIGS. 5 a-5 f. Specifically, at time T0, the entiredoping region (all regions 1-5) are covered by the impurity source 520.At time T1 (FIG. 5 b) the portion of the impurity source 520 coveringregion 5 has been removed. At time T2 (FIG. 5 c), the portion ofimpurity source 520 covering region 4 has also been removed; at time T3(FIG. 5 d) the portion of impurity source 520 over region 3 has beenremoved; at time T4 (FIG. 5 e) the portion of impurity source 520 overregion 2 has been removed; and finally, at time T5 (FIG. 5 f) the lastremaining portion of impurity source 520 over region 1 has been removed.Thus, the very heavily doped polymer film is kept in place overdifferent regions for differing amounts of time, with longer diffusiontimes producing heavier doping and, thus, a larger Δn value. Thisdiffusion doping can be performed with cladding polymer in the uncuredstate and is most accurate when the diffusion temperature is controlledto remain constant.

FIG. 4 illustrates the relationship between the time of exposure of thecladding layer to the doped source, and the Δn values. As can be seen,the longer the doping time t, the greater the Δn value.

A third embodiment is illustrated in FIGS. 6 a-6 c and FIG. 7. Referringto FIGS. 6 a-6 c, a coupler 600 has formed thereon separated polingelectrodes 1-5, one set on the left and one set on the right, as shownin FIG. 6 a. These are formed on a doped polymer layer 602 in awell-known manner. A reference electrode layer 603 is formed underneaththe doped polymer layer 602 to provide a reference electrode for theindividual poling electrodes. The doped polymer layer 602 is selectivelypoled along its length using larger and larger fields or poling voltages(E1-E5 of FIG. 7). A high poling field voltage applied to an electrodeprovides greater doping and thus greater Δn values than an areaenergized by a smaller poling voltage. Thus, by energizing electrode 1with a lower poling voltage E1 and increasing the poling voltagesequentially along electrodes 2-5, a gradually increasing Δn value, fromelectrode 1 to electrode 5, will result, as shown in FIG. 7. Thisresults in a doped core surrounded by doped-and-poled cladding.

A further embodiment is illustrated in FIGS. 8 a-8 c and FIG. 9. Thisembodiment takes advantage of the fact that an equal poling voltageapplied to doped materials of different thicknesses will result in ahigher Δn for the narrower doped material and a lower Δn for the thickerdoped material. Thus, as illustrated in FIGS. 8 a-8 c, a commonreference electrode 803 and common poling electrodes 805 (one on eachside, as shown in FIGS. 8 a and 8 b) are used to sandwich a tapereddoped material 807 (e.g., an electro-optic film deposited as acombination core/cladding layer using, for example, spray depositionthrough a moving shutter, or by continuously changing the extractionrange in a dip coating process). Because the doped material 807 istapered, continuous poling electrodes 805 (one on each side) can be usedbecause the constant applied voltage will produce a continuouslychanging field gradient (see FIG. 9) due to the changing thickness ofthe layer of doped material 807. This results in a coupler 800 that hasa first end 810 that has a higher Δn than the other end 812, withgradually increasing Δn values therebetween. This is just anotherexample of an embodiment of the present invention whereby a transitionfrom low-to-high or high-to-low Δn values can be achieved to allow asmooth transition from lightguide to transmission line.

By virtue of the present invention, transitions between lightguides andfiber can be manufactured more cheaply and easily, and the resultingtransition is smaller and less costly than those of the prior art. Whilethe above-described embodiments describe some fundamental structures andimplementation methods, it is understood that other possiblecombinations are possible and are covered by the pending claims. The keyconcept is that the index of the electro-optic polymer can be changed bydoping, and changed still further by poling the doped region. With suchability to change a given polymer's index, there is no need to formcomplex transition structures by depositing ordinary materials ofdifferent dielectric constant and thickness. Other transitions can bemade, such as vertical step-up and step-down structures, for example.With the proper choice of polymer and embedded chromophore, a reversiblemolecular polarization (or alignment) can be created, i.e., if the fieldis removed, the material reverts to its unaligned state. As the appliedfield increases, the index shifts in proportion to the field until fullalignment (maximum Δn) is reached. Such an electrically controllableindex change could be useful in directing incoming light beams todifferent detectors, thus producing a type of switch. Similarly,controlling index changes coupled with electro-optic interactions couldalso be useful in the modulation and demodulation of light beams. Manypolymers exhibit this behavior when appropriately doped with opticalchromophores, including polyimides and acrylics.

While there has been described herein the principles of the invention,it is to be understood by those skilled in the art that this descriptionis made only by way of example and not as a limitation to the scope ofthe invention. Accordingly, it is intended by the appended claims, tocover all modifications of the invention which fall within the truespirit and scope of the invention.

1. A method for producing a transition between a first element having afirst refractive index difference (Δn) percentage and a second elementhaving a second Δn percentage higher than the Δn of said first element,comprising the steps of: controlling the Δn along a length ofelectro-optic polymer to achieve a gradual transition from a low Δn to ahigh Δn along said length; and optically coupling said length ofelectro-optic polymer between said first element and said secondelement.
 2. The method of claim 1, wherein said controlling stepcomprises at least the step of: performing selective deposition on alength of undoped substrate having plural doping regions, usingincreasing doping levels with each successive doping region.
 3. Themethod of claim 2, wherein said selective deposition step is performedusing a polymer dopant.
 4. The method of claim 3, wherein said polymerdopant is deposited through photolithographic masking.
 5. The method ofclaim 4, wherein said polymer dopant is deposited by spraying saidpolymer dopant through said photolithographic masking onto saidsubstrate.
 6. The method of claim 4, wherein said polymer dopant isdeposited by dipping said photolithographically-masked substrate intosaid polymer dopant.
 7. The method of claim 4, wherein said polymerdopant is deposited using vacuum pyrolisis.
 8. The method of claim 1,wherein said controlling step comprises at least the step of: performingdiffusion doping on a length of undoped substrate having plural dopingregions, increasing the diffusion time with each successive dopingregion.
 9. The method of claim 8, wherein said step of performingdiffusion doping comprises the steps of: depositing a layer of animpurity source on said entire length of said substrate and leaving saidlayer on said substrate for a predetermined time period; removing aportion of said layer covering a first of said plural doping regions andleaving the remainder of said layer on said substrate for a secondpredetermined time period; repeating said removing step for each of saidplural doping regions until all of said layer has been removed.
 10. Themethod of claim 1, wherein said length of elector-optic polymercomprises a doped polymer core surrounded by doped and poled cladding,said doped and poled cladding comprising a layer of pluralpoling-electrodes and a common reference electrode layer, with saiddoped polymer core situated therebetween, said controlling stepcomprising at least the step of: applying a first poling voltage of afirst value to a first of said poling-electrodes; applying sequentiallyincreasing poling voltage values to each successive poling electrode.11. The method of claim 1, wherein said controlling step comprises atleast the step of: sandwiching a tapered length of doped polymer betweenfirst and second continuous poling electrodes; and applying a constantpoling voltage across said first and second continuous poling electrodesfor a predetermined time period.
 12. A transition structure situatablebetween a first element having a first refractive index difference (Δn)percentage and a second element having a second Δn percentage higherthan the Δn of said first element, said transition structure obtainableby the process steps of: controlling the Δn along a length ofelectro-optic polymer to achieve a gradual transition from a low Δn to ahigh Δn along said length; and optically coupling said length ofelectro-optic polymer between said first element and said secondelement.
 13. The transition structure of claim 12, wherein saidcontrolling step comprises at least the step of: performing selectivedeposition on a length of undoped substrate having plural dopingregions, using increasing doping levels with each successive dopingregion.
 14. The transition structure of claim 13, wherein said selectivedeposition step is performed using a polymer dopant.
 15. The transitionstructure of claim 14, wherein said polymer dopant is deposited throughphotolithographic masking.
 16. The transition structure of claim 15,wherein said polymer dopant is deposited by spraying said polymer dopantthrough said photolithographic masking onto said substrate.
 17. Thetransition structure of claim 15, wherein said polymer dopant isdeposited by dipping said photolithographically-masked substrate intosaid polymer dopant.
 18. The transition structure of claim 15, whereinsaid polymer dopant is deposited using vacuum pyrolisis.
 19. Thetransition structure of claim 12, wherein said controlling stepcomprises at least the step of: performing diffusion doping on a lengthof undoped substrate having plural doping regions, increasing thediffusion time with each successive doping region.
 20. The transitionstructure of claim 19, wherein said step of performing diffusion dopingcomprises the steps of: depositing a layer of an impurity source on saidentire length of said substrate and leaving said layer on said substratefor a predetermined time period; removing a portion of said layercovering a first of said plural doping regions and leaving the remainderof said layer on said substrate for a second predetermined time period;repeating said removing step for each of said plural doping regionsuntil all of said layer has been removed.
 21. The transition structureof claim 12, wherein said length of elector-optic polymer comprises adoped polymer core surrounded by doped and poled cladding, said dopedand poled cladding comprising a layer of plural poling-electrodes and acommon reference electrode layer, with said doped polymer core situatedtherebetween, said controlling step comprising at least the step of:applying a first poling voltage of a first value to a first of saidpoling-electrodes; applying sequentially increasing poling voltagevalues to each successive poling electrode.
 22. The transition structureof claim 12, wherein said controlling step comprises at least the stepof: sandwiching a tapered length of doped polymer between first andsecond continuous poling electrodes; and applying a constant polingvoltage across said first and second continuous poling electrodes for apredetermined time period.